So if I understand correctly, you have a webhook being called that validates the email address, right? And if the webhook's response is X, you want those leads deleted.
You can set up a trigger smart campaign for when a webhook is called and add a constraint for a specific response. Then you can have the flow step be the delete lead action.
In the Smart List section of your smart campaign, add the "Webhook is called" trigger. Select the webhook, then add the constraint for "Response" and enter the one you want to delete the leads for.